A hot strip mill acts to heat a slab as thick as several hundreds of millimeters, which is produced by continuous casting, etc., and then roll it to several to several tens of millimeters in thickness by passing it through pluralities of rolls in a roughing mill and a finishing mill successively. The finishing mill usually comprises 5 to 7 four-roll stands arranged in tandem, and a finishing mill widely used comprises seven stands. In the seven-stand finishing mill, first to third stands are called “upstream stands,” and fourth to seventh stands are called “downstream stands.”
Because rolls used in the finishing mill should withstand a thermal and mechanical load during rolling, centrifugally cast composite rolls (hereinafter referred to simply as “composite rolls”) each having a composite structure comprising an outer layer having excellent wear resistance and an inner layer integrally fused to the outer layer and having excellent toughness are used. However, because damages such as wear, surface roughening, heat cracking, etc. occur on the outer layer surface depending on a thermal and mechanical load during rolling, the composite rolls are taken out of the mill after a predetermined time period of use, to remove (cut off) damages. By damages-removing cutting, the diameter of a composite roll body becomes gradually smaller from the initial diameter to a minimum diameter usable for rolling (discard diameter). A diameter in a range from the initial diameter to the discard diameter is called an effective diameter for rolling (simply “effective diameter”).
In the finishing mill, a centrifugally cast composite roll comprising an outer layer made of high-speed steel having excellent wear resistance, and an inner layer metallurgically integral with the outer layer and made of cast iron or cast steel having excellent toughness has conventionally been used. The high-speed steel has excellent wear resistance, because high-hardness carbides such as MC-type V carbides, M6C- and M2C-type Mo carbides and W carbides, and M7C3- and M23C6-type Cr carbides, etc. are precipitated, and because decrease in the matrix hardness at high temperatures is suppressed by Mo and W. Particularly, because a thick steel sheet is rolled in the upstream stands, with little risk of damaging the outer layer by rolling a folded thin steel sheet as in the downstream stands, composite rolls whose outer layers are made of high-speed steel having good wear resistance are widely used.
Such composite roll is produced at low cost by a centrifugal casting method, which comprises casting a melt for an outer layer into a rotating centrifugal casting mold, solidifying the melt to form the outer layer on an inner surface of the mold, vertically assembling this mold with upper and lower molds to constitute a stationary casting mold, and casting a melt for an inner layer into the stationary casting mold.
JP 2-258949 A discloses a wear-resistant, centrifugally cast composite roll comprising an outer layer having a composition comprising by weight 1-4% of C, 3% or less of Si, 1.5% or less of Mn, 4% or less of Ni, 2-15% of Cr, 8% or less of Mo, 20% or less of W, 2-10% of V, and 5% or less in total of at least one selected from the group consisting of Ti, Zr and Nb, the balance being substantially Fe and inevitable impurities, the value of C %+0.4V % being 6.0 or less, and an inner layer made of cast iron or cast steel. This composite roll is subject to a hardening treatment comprising heating to a temperature equal to or higher than the transformation point of the outer layer (1000-1100° C.) and then cooling at a constant speed, and a tempering treatment at 550° C. The hardening treatment transforms the matrix of the outer layer to a hard structure such as martensite or bainite, providing the outer layer with high hardness. However, because the cooling speed in the hardening treatment becomes lower as getting deeper from the surface in a large composite roll such as a hot-rolling composite roll, the hardness of the outer layer is lower inside than on the surface.
JP 6-145887 A discloses a centrifugally cast composite sleeve roll comprising an outer layer made of a high-speed steel comprising by weight 1.8-3.0% of C, 4.0-8.0% of Cr, 2.0-8.0% of Mo, 2.0-6.0% of W, 4.0-10.0% of V, and 12.0% or less of Co, the balance being substantially Fe, and an inner layer made of spheroidal graphite adamite comprising by weight 1.0-2.0% of C, 1.0-3.0% of Si, 0.2-1.0% of Mn, and 0.3-1.5% of Ni, the balance being substantially Fe. This composite sleeve roll is subject to a hardening treatment from a high temperature of 1000-1200° C. In this composite sleeve roll, the outer layer has substantially constant hardness up to the depth of about 100 mm from the surface.
As described above, in conventional centrifugally cast composite rolls subject to a hardening treatment after casting, the hardness of the outer layers is lower inside than on the surface, or substantially constant to some depth. It has thus been widely appreciated by those skilled in the art that in conventional centrifugally cast composite rolls, the hardness of the outer layers decreases at it gets deeper.
In each of upstream and downstream stands in a seven-stand finishing mill, composite rolls of the same material are used usually. For example, in the upstream stands comprising the first to third stands, it is likely that composite rolls of the initial diameter are used in the foremost first stand in which a thick steel sheet tends to be bit, composite rolls with an effective diameter reduced by damages-removing cutting are used in the second stand, and composite rolls with an effective diameter further reduced by damages-removing cutting are used in the third stand. Thus, composite rolls with effective diameters reduced by damages-removing cutting are transferred from the first stand to the second stand, and then to the third stand.
Composite rolls in the first stand are subject to deep heat cracks by heat shock because it first comes into contact with a high-temperature sheet to be rolled. Because heat cracks cause the surface roughening of a composite roll, resulting in a deteriorated surface quality of a rolled sheet, a large amount of damages-removing cutting is conducted to remove heat cracks. A poorly rolled tip portion of the steel sheet passing through the roughing mill is cut off by the crop shears, to prevent the biting of a steel sheet into the finishing mill and the deterioration of surface quality. However, shear-cutting causes folding and the formation of oxide scales in the tip portion of the steel sheet, damaging composite rolls in the first stand.
Though composite rolls in the second and third stands downstream of the first stand are free from damages due to folding and oxide scales, they are required to have small surface roughness (smooth surfaces) because they are arranged upstream of the fourth to seventh stands. Namely, composite rolls with large effective diameters used in the first stand are required to have resistance to scratches due to folding and oxide scales (surface-roughening resistance), and composite rolls with small effective diameters used in the third stand are required to have smooth surfaces. However, all of these requirements variable depending on its effective diameter would not be able to be met by one composite roll.